1. Field Of The Invention
The present invention is concerned with improvements in catalysts useful for the treatment of gases to reduce contaminants contained therein. More specifically, the present invention is concerned with improved catalysts of the type often used to treat automotive exhaust gases, such as those often referred to as "three-way conversion" or "polyfunctional" catalysts. These catalysts have the capability of substantially simultaneously catalyzing the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides.
2. Background and Prior Art
Catalysts as described above find utility in a number of fields including the treatment of the exhaust from internal combustion engines, such as automobile and other gasoline-fueled engines. Emissions standards for unburned hydrocarbons, carbon monoxide and nitrogen oxides contaminants have been set by various governments and must be met, for example, by new automobiles. In order to meet such standards, so-called catalytic converters containing a suitable catalyst are emplaced in the exhaust gas lines of internal combustion engines to promote oxidation of the unburned hydrocarbons and carbon monoxide and the reduction of nitrogen oxides. If the engine operation is too rich in fuel to provide sufficient oxygen inherently in the exhaust gas, oxygen (air) may be introduced into the exhaust gas as required. The use of separate catalyst beds to promote, respectively, oxidation and reduction, is known and it is also known to use a catalyst system combined in a single bed to substantially simultaneously promote both the oxidation and reduction reactions. A great deal of activity has been engendered in the field in an attempt to economically produce catalysts which exhibit good activity and long life in promoting such three-way conversion of hydrocarbons, carbon monoxide and nitrogen oxides to carbon dioxide, water and nitrogen. Three-way conversion catalysts usually require that the ratio of air to fuel ("A/F ratio") introduced into the engine whose exhaust gas is being treated not exceed a narrow deviation from the stoichiometric ratio in order to achieve optimal, sustantially simultaneous reduction/ oxidation ("redox") reactions. For purposes of promoting three-way conversion, catalysts comprising one or more platinum group metals dispersed upon a high surface area support material are well known in the art. The support material may comprise a high surface area alumina coating carried on a carrier substrate such as a monolithic carrier comprising a refractory ceramic honeycomb structure, as is well known in the art.
Such high surface area alumina support materials, sometimes referred to as "active" or "activated" alumina, typically exhibit a BET surface area in excess of 60 m.sup.2 /g, e.g., up to about 150 or 200 m.sup.2 /g or more. Activated alumina is usually a mixture of the gamma and delta phases of alumina, but may also contain substantial amounts of other phases such as amorphous, eta, kappa and theta alumina phases. Typical catalyst compositions comprise a minor amount of a platinum group metal component such as platinum or palladium, optionally including one or more of rhodium, ruthenium and iridium dispersed on an activated alumina support material. The alumina support material may be carried on a carrier substrate, such as a honeycomb type substrate, having a plurality of fine gas flow passages extending through it. In order to facilitate coating these fine passages, a slurry of exceedingly fine particles of activated alumina, sometimes referred to as a "washcoat", may be prepared and applied to the substrate. Alternatively, or in addition, an activated alumina precursor may be applied to the substrate and then converted to activated alumina. In any case, the resultant high surface area support alumina material enhances the catalytic activity of the composition by enabling dispersal of the catalytically active platinum group metal component on the high surface area alumina washcoat material instead of directly upon a low surface material such as the carrier substrate.
A common deficiency associated with such catalyst compositions is thermal degradation of the activated alumina support material by extended exposure to high temperatures encountered in the treatment of the gases, such as internal combustion engine exhaust gases. In a moving vehicle for example, exhaust gas temperatures can reach 1000.degree. C. Such elevated temperatures cause the activated alumina support material to undergo a phase transition to lower surface area alumina, with accompanying volume shrinkage, especially in the presence of steam, whereby the catalytic metal become occluded in the shrunken support medium with a loss of exposed catalyst surface area and a corresponding decrease in catalytic activity. It is a known expedient in the art to stabilize the alumina against such thermal degradation by the use of stabilizer materials such as zirconia, titania, alkaline earth metal oxides such as baria, calcia or strontia or, most usually, rare earth metal oxides, for example, oxides of cerium, lanthanum, neodymium, praseodymium and mixtures of two or more rare earth metal oxides, including the commercially available mixtures of rare earth metal oxides. For example, see U.S. Pat. No. 4,171,288 of Carl D. Keith, et al.
The use of support materials other than activated alumina are known. For example, because rhodium interacts deleteriously with gamma alumina, particularly under lean exhaust conditions, the art has suggested substituting materials such as alpha-alumina (U.S. Pat. No. 4,172,047) or zirconia (U.S. Pat. No. 4,233,189) as support materials which are not interactive with rhodium. However, alpha-alumina and zirconia are relatively low surface area materials, which is disadvantageous inasmuch as catalyst durability in such gas purification use depends to a certain extent on the surface area of the support material.
U.S. Pat. No. 4,539,311 discloses a lead-resistant catalyst for treating motor vehicle exhaust fumes, which catalyst may comprise a honeycomb support coated with an alumina washcoat catalytic coating. A high surface area alumina which may incorporate ceria is impregnated first with a barium moiety, such as aqueous solution of a barium compound, e.g., barium nitrate, which decomposes to produce barium oxide on firing at over 400.degree. C. After such firing the catalyst is impregnated with a dispersion of a platinum group metal moiety, such as by soaking the alumina in an aqueous solution of a metal compound, e.g., chloroplatinic acid, which on firing at over 400.degree. C. decomposes to leave behind either the platinum group metal or a compound which converts to the metal when the catalyst is placed in use.
U.S. Pat. No. 4,294,726 discloses a three-way conversion catalyst composition containing platinum and rhodium supported on activated alumina as well as cerium oxide, zirconium oxide and iron oxide in stated proportions. The catalyst is obtained by impregnating a gamma alumina carrier material with an aqueous solution of cerium, zirconium and iron salts or by mixing the alumina with the oxides of cerium, zirconium and iron. The material is then calcined at 500.degree. to 700.degree. C. in air. The addition of ceria-zirconia-iron oxide is followed by impregnating the treated carrier material with aqueous salts of platinum and rhodium and then treating the impregnated material in a hydrogen-containing gas at a temperature of 250.degree.-650.degree. C. Thermal stabilization of the alumina may be separately carried out by impregnation of the alumina with a solution of calcium, strontium, magnesium or barium compounds. The addition of ceria-zirconia-iron oxide is stated to enable operation at leaner A/F ratios.
U.S. Pat. No. 3,966,790 discloses catalysts having good high temperature stability and which include a platinum group metal deposited on an activated alumina coating which is stabilized against thermal degradation by dispersing therein selected metal oxides which serve as stabilizers. The metal oxides may be selected from Group IVA metals (e.g., silicon and tin); Group IVB metals (e.g., titanium, zirconium, hafnium and thorium); Group IIA metals, i.e., alkaline earth metals (e.g., beryllium, magnesium, calcium and barium); and Group VIB metals (e.g., chromium, molybdenum and tungsten). The composites may be added by impregnating the activated alumina with solutions of soluble precursors of the stabilizers or by co-precipitating the alumina and stabilizers from aqueous solutions. High surface area colloidal silica is stated (column 4, line 56 et seq) to be a useful additive; a silica sol containing silica in a particle size of about 15 millimicrons is stated to be particularly useful.
The prior art has thus attempted to ameliorate thermally induced phase transformation of the alumina support material from high surface area to low surface area phases (e.g., from gamma to alpha alumina) by stabilization of the support material. Conventionally, this is accomplished by impregnation of the alumina support material with a solution of a metal compound precursor of the desired stabilizer oxide. Upon drying and calcining of the impregnated activated alumina, an alumina support material containing dispersed stabilizer is obtained. As noted above, among such stabilizers are alkaline earth metal oxides and rare earth metal oxides. It is believed that the stabilizer cations located at the surface of the alumina particles readily diffuse into the surface lattices of the alumina, providing interaction of the stabilizer ions with the alumina when sufficiently high temperature, e.g., of 800.degree. C. or more, are reached. In this way, thermally induced phase transformation of the activated alumina support to lower surface area phases such as alpha alumina can be effectively retarded.
Conventional methods of preparing an alumina support material washcoat include ball milling of the stabilized activated alumina in an acidified medium to reduce its particle size in order to form a coatable slurry or washcoat of the alumina. However, the acid ball milling results in an appreciable loss in efficiency of the stabilization treatment, the presence of acid in the milling process apparently causing the stabilizers to form soluble ionic species, colloidal particles or hydrated gel type structures which render the stabilization less effective.
The art also shows the inclusion in catalyst compositions comprising activated alumina support materials and catalytic and/or catalyst modifier materials, e.g., a metal oxide, added in bulk form, that is, added as solid particulate materials rather than by impregnation of the alumina with a solution of a suitable precursor. For example, bulk ceria or bulk zirconia may be added to the composition as shown in U.S. Pat. No. 4,624,940.